Disc drive transducer deadbeat settle method utilizing interval correction

ABSTRACT

A method for settling a transducer (18) of a hard disc drive (10) on a target track (30) on a rotating disc (12) of the disc drive at the end of a seek to the target track in a determined time period. The time period is divided into two deadbeat intervals(58, 60), each of which is divided into a plurality of time intervals in each of which the location of the transducer with respect to the target track is measured (80) and an accepted value of transducer velocity is measured (82) or determined from successive locations of the transducer. In the first time interval of the each deadbeat interval, a deadbeat value of a control signal is determined (88) on the basis of ideal deadbeat settle of the transducer to the target track and outputted to a power amplifier (36) that drives the actuator (40) on which the transducer is mounted for radial movement of the transducer across the disc. In subsequent intervals of each deadbeat interval, an interval correction (102), determined in relation to the difference between a projected velocity determined from the deadbeat value of the control signal and the accepted velocity of the transducer, is added to the deadbeat value and outputted (94) to the power amplifier. A second interval correction 132, 136, 138 can be added to the deadbeat value to correct for a long power amplifier time constant.

TECHNICAL FIELD

The present invention relates generally to improvements in disc driveservo methods, and, more particularly, but not by way of limitation, toimprovements in methods for settling a disc drive transducer on a targettrack defined on a rotating disc of a disc drive at the end of a seek tothe target track.

BACKGROUND ART

In a disc drive for storing computer files, the files are stored inmagnetizable surface coatings on rotating discs by magnetizing thecoatings in a pattern that reflects bits of data of which the file iscomprised. To this end, a disc drive is comprised of a plurality ofread/write transducers, supported in close proximity to the discsurfaces by an electromechanical actuator, that receive electricalcurrents reflective of the data bits to produce magnetic fields thatmagnetize the surface coatings in either of two directions as thecoatings pass the transducer. Subsequently, files can be read by usingthe transducers to detect magnetic fields produced adjacent the discsurfaces by the magnetization of the coatings.

In order to record or retrieve a file, concentric data tracks aredefined on the disc surfaces and records are maintained in a fileallocation table to identify the surface, or surfaces, that contain aparticular file and the data tracks on those surfaces at which the fileis located. During writing and reading of a file, the transducer that isto read or write data is maintained in radial alignment with a selectedtrack by a servo system that is operated in a track following mode byreading servo patterns magnetically prerecorded on one or more of thedisc surfaces and continually maintains the alignment between thetransducer and the selected track. The servo system is also operable ina seek mode for moving the transducers from a track currently beingfollowed to a target track that contains, or is to receive, a new file.

The widespread use of disc drives to store computer files stems from twobasic disc drive characteristics arising from the above recitedconstruction and manner of operation: large data storage capacity andshort file access times. Positioning of the transducers by a servosystem permits the data tracks to be closely spaced with the result thata disc drive can store a tremendous amount of user data in a smallvolume. Average random access time is a weighted time average based uponthe time to seek from one track to another for all possible seeklengths. The probability of shorter seek lengths is higher than thelarge seek lengths. Further, a one track seek has the highestprobability and therefore the time for a one track seek has the heaviestweight. Settle time is a larger percentage of seek time for the shorterseek lengths and in general, the shorter the seek length the higher thepercentage of the settle time relative to total seek time. Therefore, asignificant reduction in settle time can result in a significantreduction in seek time for the shorter seek lengths which in turn wouldreduce average random access time. It is the reduction in random averageaccess time that the present invention addresses.

Seeks from one trick to another are usually realized under some form ofvelocity control, during which the transducers follow a velocitytrajectory determined by a velocity profile table contained in the servomicroprocessor memory. The velocity profile varies the velocitytrajectory as a function of the distance to the target track. During theseek, the location and velocity of the transducers are repetitivelysampled and the transducers are accelerated in relation to thedifference between the actual transducer velocity and the profilevelocity for the present distance remaining in the seek. The velocityprofile is selected to cause the transducers to undergo a rapidacceleration at the initiation of the seek and to decelerate to thetarget track at the end of the seek by requiring the profile velocity tobe large for large distances from the target track and to decrease tozero as the distance to the target track approaches zero.

While this general approach has worked well, it suffers from limitationsthat give rise to problems in the settling of the transducers on thetarget track at the end of a seek. The velocity profile is stored as aset of discrete values in a look-up table and the number of values thatmay be stored is limited by the amount of memory that can be allocatedto velocity control during a seek. As a result, the profile velocitiesare only approximately defined. Moreover, the difference in velocitysteps of the profile and the time between steps increases as the speedof the transducers becomes small at the end of a seek. Further, theacceleration of the transducers is limited by the amount of power thatcan be supplied by a power amplifier that drives the actuator toaccelerate the transducers so that the transducers will onlyapproximately follow the velocity profile. The net result is that thetransducers reach the vicinity of the target track with a range ofvelocities that can complicate the problem of settling the transducerson the target track; that is, bringing the transducers to rest on thetarget track, so that writing or reading may be commenced. Consequently,settling times, usually effected under position control of thetransducers, can be undesirably extended to increase the time requiredto access a track on which a file is to be read or written.

In order to minimize the time required for the transducer to settle on atarget track, it has been proposed to use a deadbeat settle approach inwhich the transducers are accelerated or decelerated in a succession ofdeadbeat intervals in response to control signals that are determined bythe initial conditions with which the transducers enter a settle controlregion defined about each track and by the requirement that the settleend with the transducer on track with zero velocity. However, disc drivepower amplifier limitations require that the deadbeat intervals be maderelatively long to insure that the acceleration or deceleration of thetransducers required in a deadbeat settle can be achieved. As aconsequence, deadbeat settle for a disc drive requires that the servocontrol loop be open for long periods of time.

Long open loop periods reduce the ability of a disc drive to reducedisturbances. In particular, the forces acting on the actuator duringthe settle period can have an appreciable effect on the performance of adeadbeat controller when the deadbeat period is relatively long. As isknown in the art, the actuator is subject to a number of disturbingforces; for example, windage forces exerted on the actuator by airswirled by the discs, and these forces, if not compensated, can have anappreciable effect over a long deadbeat settle interval that willprevent the deadbeat settle scheme from accomplishing its goal ofbringing the transducer to rest at the target track. Thus, prior to thepresent invention, the problem of minimizing the time for settling thetransducers on a target track at the end of a seek in the presence ofexternal disturbances has not been effectively solved.

DISCLOSURE OF THE INVENTION

The present invention provides a settle method which retains theadvantages of deadbeat settle while compensating for the disturbancesthat could cause a pure deadbeat approach to deviate greatly from theideal response. To this end, the settle method of the present inventioncontemplates that settling of the transducers will be effected, as inthe pure deadbeat approach, in a plurality of deadbeat intervals whichare each comprised of a plurality of time intervals in whichcompensation is effected for disturbances the actuator may experienceduring the settling of the transducers. More particularly, in the firsttime interval of each deadbeat interval, control signals outputted fromthe servo microprocessor to the power amplifier that drives the actuatorare determined on the basis of deadbeat settle; that is, in accordancewith a control equation that satisfies deadbeat settle conditions.Subsequently, in each remaining time interval of each deadbeat interval,a projected velocity; specifically, the velocity the transducers wouldattain at the beginning of the time interval under deadbeat settleconditions, is calculated and compared to an accepted value of thetransducer velocity that can be determined in any convenient way. Forexample, should the disc drive comprise a velocity transducer, theoutput of such transducer, inputted at the beginning of the timeinterval can be used as the accepted velocity. However, as will bediscussed more fully below, the invention is not limited to disc drivesincluding actuator velocity transducers. The control signal outputted tothe power amplifier is then adjusted from the value determined fordeadbeat settle in the first time interval of the deadbeat interval byadding an interval correction that is proportional to the differencebetween the projected and accepted values of the transducer velocity.Consequently, the transducer is caused to substantially follow adeadbeat trajectory to reach substantially the final state that would beachieved in an ideal deadbeat settle; that is, on track with zerovelocity, at the end of the final deadbeat interval.

An important object of the present invention is to reduce access time oftracks along which data is stored in a disc drive.

Another object of the invention is to minimize track access time byminimizing the time required for settling a transducer on a disc drivedata track at the end of a seek to the data track.

Still a further object of the invention is to provide an effectivemethod for achieving the advantages of deadbeat settle in a disc drivewhile compensating for disturbances that would interfere with settle ina deadbeat approach.

Other objects, features and advantages of the present invention willbecome apparent from the following detailed description when read inconjunction with the drawings and appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a typical disc drive in which the method ofthe present invention is practiced.

FIG. 2 is a comparison of the transducer velocity versus time for idealdeadbeat settle of the transducer and settle in accordance with thepresent invention.

FIG. 3 is a comparison of the control signal versus time for idealdeadbeat settle and settle in accordance with the present invention.

FIG. 4 is a flow chart of the servo system microprocessor seek programfor moving transducers of the disc drive to a target track at which afile is to be stored or from which a file is to be retrieved.

FIG. 5 is a flow chart illustrating the method for settling a transduceron a target track in accordance with the present invention.

FIG. 6 is a flow chart illustrating a modification of the method forsettling a transducer on a target track in accordance with the presentinvention.

DESCRIPTION OF THE DISC DRIVE

Referring to FIG. 1, shown therein and designated by the generalreference numeral 10 is a block diagram of a disc drive in which themethod of the present invention might be practiced. More particularly,FIG. 1 illustrates features, typically found in disc drives, that areused in carrying out the inventive method in a manner to be describedbelow. As is known in the art, such features vary from one disc drive toanother so that FIG. 1 is not intended to illustrate a specific discdrive. Rather, it is intended to generally present disc drive featuresin order to provide a basis for describing the manner in which thefeatures of any disc drive may be utilized in the practice of the methodof the present invention.

In general, the disc drive 10 is comprised of a plurality of discs 12,14 that are mounted on a spindle 16 for rotation of the discs about theaxis of the spindle 16. As is conventional in the art, the discs 12, 14have magnetizable surface coatings to permit data received from a hostcomputer (not shown) to be written to the discs in the form ofmagnetized cells by read/write transducers, such as the transducer 18 inFIG. 1, that "fly" over the disc surfaces in close proximity thereto. Tothis end, the disc drive 10 is comprised of a read/write circuit 20 thatreceives a file to be stored from a host interface 22 and transfers thefile, after encoding, to the read/write transducer 18 that is to writethe data to a disc 12. Subsequently, the file can be read by thetransducer 18, decoded by the read/write circuit 22 and returned to thehost interface 22. Control and coordination of the operation of the discdrive 10 is effected by commands issued by a system microprocessor 24 tothe host interface 22 and, via a system logic circuit 26, to theread/write circuit 20.

Similarly, the system microprocessor 24 issues commands to a servomicroprocessor 28 that controls the location of the transducer 18 sothat a file can be written to, or retrieved from, any location on anydisc surface. More particularly, the disc surfaces are organized intoconcentric data tracks, such as the data track 30 on the upper surface32 of disc 12, and the servo microprocessor 28 is programmed to receivecommands from the system microprocessor 24, via the system logic IC, andgenerate commands and a control signal that will effect seeks of theread/write transducer 18 from one data track to another and control theposition of the transducer 18 to maintain data track following once aseek has been completed. Typically, the commands and the control signalare provided to a servo logic IC 34 and the control signal, transmittedto a power transconductance amplifier 36, results in the passage of acurrent through a coil 38 of an actuator 40 on which the read/writetransducers, such as the transducer 18, are mounted via arms 42 thatextend into the stack of discs 12, 14 mounted on the spindle 16. Thecoil 38 is immersed in a magnetic field so that the control signal istranslated, first, into a current through the coil 38 and, consequently,into a force that can be used to accelerate the read/write transducers18 in either radial direction across the disc surfaces.

To enable the servo microprocessor to generate the control signalrequired to effect a specific movement of the read/write transducers 18,for track seeking or following, it is common practice to write servopatterns to one or more disc surfaces and read the patterns to generatetransducer position location information that is transmitted to theservo microprocessor 28. A common practice with respect to thegeneration of the position information, illustrated in FIG. 1, is todedicate one surface 44 of one disc, the disc 14 in FIG. 1, to servocontrol of the actuator 40 and write the servo patterns to the dedicatedservo surface at the time the disc drive 10 is manufactured. Thesepatterns are then read by a servo transducer 46, supported in closeproximity to the dedicated servo surface 44 by the actuator 40 and asupport arm 47, to generate signals indicative of the servo transducerlocation for transmission to the servo microprocessor 28 as indicated bythe signal path 48 in FIG. 1. A suitable format for the servo patternswritten to a dedicated servo surface of a disc of a disc drive has beenillustrated in U.S. Pat. No. 5,262,907, entitled "Hard Disc Drive withImproved Servo System" issued Nov. 16, 1993 Duffy et al. Moreover, andas also taught in U.S. Pat. No. 5,262,907, the output of the poweramplifier 36; that is, the current through the actuator coil 38, may betransmitted to the servo microprocessor 28, as symbolically indicated bythe signal path 50 in FIG. 1, to provide further information withrespect to the mechanical state of the servo transducer 46.

As the above indicates, control of the position and of movement of theread/write transducers 18 in a disc drive having a dedicated servosurface 44 is effected by controlling the position and movement of theservo transducer 46. To this end, servo tracks, one of which has beenindicated at 52, are defined by the servo patterns on the dedicatedservo surface and the data and servo tracks are organized into cylinderswhich each contain a servo track and a plurality of data tracks, one oneach data surface. (The cylinder containing the data track 30 and servotrack 52 has been indicated at 54 in FIG. 1.) A more completedescription of a disc drive servo system having the described featurescan be found in the aforementioned U.S. Pat. No. 5,262,907, theteachings of which are hereby incorporated by reference.

A useful feature that has also been described in U.S. Pat. No. 5,262,907is a capability for adjusting the position information received from theservo transducer 46 to compensate for mechanical offsets between theservo transducer 46 and each read/write transducer 18. As taught by suchpatent application, predetermined values of the offsets can besubtracted from the servo transducer position information in the servologic IC 34, by outputting the offset to a difference amplifier (notshown) that converts signals received from the servo transducer to aposition error signal that is transmitted to the servo microprocessor28. As will be clear to those of skill in the art, it is the position ofthe read/write transducers 18, instead of the servo transducer 46, thatis to be controlled during track following and during settle of thetransducers on a selected track. The use of this feature will bediscussed below. However, it will be recognized that such feature in nota necessity for the practice of the present invention; the position ofeach read/write transducer 18 can be calculated from the position of theservo transducer 46 once the offsets have been measured. A particularlyuseful method for determining and using the offsets has been describedin U.S. patent application Ser. No. 08/060,858 entitled "Improved DataTrack Following Method for Disc Drives" filed May 12, 1993 by Hampshireet al.

A final feature of disc drives which, if present in a disc drive, findsuse in the present invention is a velocity transducer 56, indicated indashed line in FIG. 1, that is mechanically linked to the actuator 40and provides a signal indicative of the rate of rotation of the actuator40 to the servo logic IC 34.

IDEAL DEADBEAT SETTLE

As has been noted above, the method of the present invention utilizescontrol signals that are determined, in part, in accordance with anideal deadbeat approach to settling of the transducer 18 on data tracksat the end of a seek. Accordingly, it will be useful to briefly reviewthe deadbeat settle strategy to provide a basis for description of thesettle method of the present invention. FIGS. 2 and 3, which present thevelocity and acceleration of the transducer 18 during an ideal secondorder deadbeat settle have been presented for this purpose and, further,for the purpose of more fully bringing out the inventive method to bediscussed below. While a second order deadbeat settle has beenillustrated in FIGS. 2 and 3, it will be recognized by those of skill inthe art that the present invention is not limited to the determinationof control signals with respect to a second order deadbeat settle.Rather, second order deadbeat settle has been presented in FIGS. 2 and 3in the spirit of providing a concrete example that will facilitate anunderstanding of the present invention.

The deadbeat settle strategy contemplates that a control object, forexample, a read/write transducer of a disc drive, will be brought torest at a specific location in a plurality of deadbeat intervals equalin number to the order of the particular deadbeat control realizationthat is employed. Thus, in a second order deadbeat realization, thecontrol object is brought to rest in two deadbeat intervals that havebeen indicated at 58 and 60 respectively in FIGS. 2 and 3. In eachdeadbeat interval, the control object is uniformly accelerated ordecelerated so that a graph of the velocity of the control object as afunction of time would have the form of two straight lines as indicatedby the lines 62 and 64 in FIG. 2. Similarly, in each deadbeat interval,the acceleration is proportional to a control signal so that, during anideal second order deadbeat settle of a control object, the controlsignal would have two successive constant values as indicated by thedashed lines 66 and 68 in FIG. 3.

In the case in which the control signal is outputted to atransconductance amplifier, as in the present invention, to generate acurrent that is passed through a coil immersed in a magnetic field, therelationship between the control signal and the acceleration can beexpressed by the equations:

    A(N)=Km I(N)                                               (1)

    and

    I(N)=Ki U(N)                                               (2)

where A(N) is the acceleration of the control object in the Nth deadbeatinterval, I(N) is the current through the coil in the Nth deadbeatinterval, U(N) is the value of the control signal in the Nth interval,Km is a constant determined by the mechanical characteristics of thesystem being controlled; for example, a disc drive actuator, and Ki isthe transconductance of the amplifier.

Deadbeat settle is then, ideally, realized by relating the controlsignal in each deadbeat interval to the state of the control object atthe start of the interval; that is, to the location and velocity of thecontrol object at the start of the interval. In the case of a secondorder deadbeat realization, this relationship can be expressed as

    U(N)=-Kp X(N)-Kv V(N),                                     (3)

where X(N) and V(N) define the state of the system at the start of theNth deadbeat interval and Kp and Kv are control constants determined bythe requirement that the state of the system at the end of the Nthinterval be zero; that is, that the control object come to rest withzero position error at the location from which the position X of thecontrol object is measured. In the case that has been illustrated inFIGS. 2 and 3, Kp and Kv can be determined from the equations

    2-Kv Ki Km Td-Kp Ki Km Td.sup.2 /2=0                       (4)

    and

    1-Kv Ki Km Td+Kp Ki Km Td.sup.2 /2=0                       (5)

where Td is the duration bf the deadbeat interval. Additional deadbeatconstants are similarly derived for higher order deadbeat strategies.

The present invention contemplates the use of deadbeat constants Kp(DB)and Kv(DB), in a manner that will be discussed below, that aredetermined on the basis of the ideal deadbeat strategy outlined aboveand, in such case, the constants Kp(DB) and Kv(DB) would have the valuesKp and Kv determined in accordance with equations (4) and (5).Similarly, should it be desired to adapt a higher order deadbeat settlestrategy to the purposes of the present invention, additional deadbeatconstants would be selected to be deadbeat constants determined in theconventional manner. While, in the presently preferred practice of thepresent invention, only the constants Kp(DB) and Kp(DB) are utilized,the inventive method is not so limited. Rather, it is contemplated thatadditional deadbeat constants can be determined and utilized in thepractice of the invention.

While, in principle, a deadbeat strategy as described above can beutilized to settle a read/write transducer on a selected data track,practical problems make a pure deadbeat scheme undesirable. For example,the power amplifiers of disc drives commonly have limited power handlingcapabilities to meet a disc drive power budget imposed by theenvironment; i.e., a computer, in which disc drives are used.Consequently, the acceleration of the transducers is limited so that thedeadbeat intervals must be extended to avoid excessive control effortthat would be required for shorter intervals that might be utilized inhigh order deadbeat realizations. However, the servo loop is open duringeach deadbeat interval so that forces, other than the force exerted bypassage of a current through the actuator coil to accelerate theactuator, can give rise to large errors in the final mechanical state ofthe transducer. Such errors require time to correct and, accordingly,increase the access time for a move to a selected track. While thedisturbances might be modeled and compensated in a deadbeat settlewithout using a lengthy deadbeat interval, such an approach can createdemands on the servo microprocessor that are beyond its capabilities.Thus, a pure deadbeat approach to disc drive transducer settle, whilefeasible, is unattractive. The present invention provides an alternativesettle method that provides all of the advantages of a pure deadbeatstrategy while avoiding the major problems of the strategy. Such method,in two embodiments, will be discussed with respect to FIGS. 5 and 6.

MODES FOR CARRYING OUT THE INVENTION

Referring initially to FIG. 4, shown therein is a flow chart for a seekthat has been adapted to terminate in the settle method of the presentinvention. To this end, the servo microprocessor 28 will respond to acommand from the system microprocessor 24 to access a particular datatrack for storage or retrieval of a file by a selected read/writetransducer 18 by initializing a deadbeat interval index N to zero at astep 70. Such index identifies each of a plurality of deadbeat intervalsthat, in the practice of the present invention, have durations selectedon the basis of power amplifier 36 capabilities to include a pluralityof time intervals during which the mechanical state of the transducer 18which is to be moved to the data track can be determined during settleof the transducer 18 on the data track. Thus, for example, in the discdrive servo system described in the aforementioned U.S. Pat. No.5,262,907, servo operations are carried out during interrupts of theservo microprocessor 28 that occur at equal time intervals and eachdeadbeat interval would be selected to be a fixed number of timeintervals between successive interrupts. Alternatively, the timeintervals which make up each deadbeat interval can be a servo routineloop time in which servo control is repetitively asserted by measuringparameters associated with the present state of the disc drive servosystem and calculating a value of a control signal that is outputted tothe power amplifier 36 to change such state in a desired way. Thegrouping of a plurality of time intervals to form a deadbeat intervalhas been indicated by the numbered time divisions in the graphs of FIGS.2 and 3.

Following initialization of the deadbeat index N, a time interval indexQ is initialized to zero at step 72. Such index identifies each timeinterval of each of the deadbeat intervals during which the settle ofthe transducer 18 on a target track is to be effected.

As noted above, the servo system that has been described in theaforementioned U.S. Pat. No. 5,262,907 has a capability of providing anoffset to the servo logic IC 34 to compensate for misalignment betweenthe servo and read/write transducers and, in such case, the offsetappropriate to the read/write transducer that is to be settled on a datatrack for reading or writing a file is outputted at a step 74 shown inFIG. 4. The servo microprocessor 28 then enters a loop indicated by thedecision block 76 and operations block 78 in FIG. 4. After checking todetermine whether the transducer 18 has reached a location, measuredfrom the target track, at which settle is to commence, step 76, theservo microprocessor will execute a conventional velocity control, orseek, routine, block 78, that will result, for a number of executions ofthe routine, in the acceleration of the transducer 46 and, consequently,the transducer 18 away from the track currently being followed and,subsequently, in the deceleration of the transducers toward the targettrack to which a file is to be written or from which a file is to beread. The manner in which a seek is carried out using velocity controlof the movement of the transducers has been described in theaforementioned U.S. Pat. No. 5,262,907 and need not be furtherconsidered herein. As the seek proceeds, the transducer 18 willeventually be brought to a location from which a settle of thetransducer 18 on the target track can be commenced and, at such time,the servo microprocessor 28 will exit from the seek routine loop to thesettle routine that is presented, for the embodiment of the inventionunder consideration, in FIG. 5 as indicated by the connector labelled Ain FIGS. 4 and 5. Suitably, the exit to the settle method can beselected to occur when the transducer 18 reaches a distance of one trackspacing from the target track or enters a fine control region definedabout each of the data tracks, often of width of one half of the spacingbetween successive data tracks.

Before turning the settle method of the present invention, it will beuseful to note a feature of the seek routine that has been illustratedin FIG. 4. As is known in the art, seeks are most often made betweensuccessive cylinders of data and servo tracks. It is contemplated that,for one track seeks, the settle method of the present invention will becarried out without the execution of velocity control routines thatwould be executed for lengthy seeks. Such immediate entry of the servomicroprocessor 28 into the settle mode, effected by selecting a settlerange of one track spacing and preceding the velocity control block 78of FIG. 4 by the decision block 76 in FIG. 4, is particularly wellsuited for a disc drive servo system having the features that have beendescribed in the aforementioned U.S. Pat. No. 5,262,907. As describedtherein, each data track is addressed so that the location of thetransducer 18 with respect to the target track can be determined for anylocation of the transducer 18 on the disc surface 32.

Referring now to FIG. 5, shown therein is flow chart of the servomicroprocessor 28 programming by means of which one embodiment of thesettle method of the present invention is carried out. For purposes ofexample, the flow chart in FIG. 5 contemplates that the deadbeat settlestrategy that has been utilized to determine the deadbeat constantsemployed in the present invention is a second order strategy so that thesettle of the transducer 18 on a target track will be accomplished intwo deadbeat intervals. Each of these deadbeat intervals is comprised ofa plurality, Qmax, of time intervals in which the steps illustrated inFIG. 5 are carried out. These time intervals can each be defined to be aselected loop time entered into the servo microprocessor 28 or, in adisc drive having a servo system as described in the aforementioned U.S.Pat. No. 5,262,907, to be one interrupt of the servo microprocessor 28.In any event, the present invention contemplates that the transducer 18will be brought substantially to rest at substantially the target trackin a number of time intervals that is determined by the product of thenumber of time intervals in a deadbeat interval and the number ofdeadbeat intervals selected to determine the deadbeat constants that areutilized in carrying out the transducer settle method of presentinvention. As noted above, the number of deadbeat intervals is afunction of the order of the system.

In initial steps of each time interval of each deadbeat interval bymeans of which the settle method of the present invention isimplemented, parameters indicative of the present mechanical state areinputted from the servo logic IC 34 to provide a basis for determiningthe control signal to be outputted to the power amplifier 36 via theservo logic 34. Such parameters include a position error X, step 80,that provides a measure of the distance between the transducer 18 andthe target track and other parameters that depend upon the features thathave been incorporated in the disc drive in which the method of thepresent invention is practiced. In particular, if the disc driveincludes a velocity transducer 56, the velocity of the transducer 18will also be inputted. Similarly, the current through the coil 38 at thebeginning of the time interval is inputted if the disc drive in whichthe invention is practiced has a capability for measuring the coilcurrent and providing such current to the servo microprocessor 28.

These additional parameters can be used to determine an acceptedvelocity for the transducer 18 at the beginning of the time interval ina step that has been generally indicated at 82 in FIG. 5. Thus, if thedisc drive includes a velocity transducer 56, the step 82 would be theinput of the velocity measurement made by the velocity transducer 56. Insuch case, the accepted velocity would be the actual velocity of thetransducer 18 as measured by the velocity transducer 56.

However, the accepted velocity need not be the actual velocity of thetransducer 18 and the disc drive 10 need not include a velocitytransducer 56 for the practice of the present invention. As will beclear to those of skill in the art, the actual final state of thetransducer 18 need not be the ideal deadbeat settle final state in whichthe velocity of the transducer 18 and the position error for thetransducer 18 are zero. Indeed, such conditions are not maintainedduring track following by the transducer 18 while files are written toor read from a data track. Thus, the method of the present inventiondoes not have as its purpose the reduction of the final state of thetransducer 18 to the deadbeat ideal; rather, the present inventionachieves a practical final state for the transducer 18 in which thetrack following mode of operation of the disc drive servo system,carried out during reading and writing of files, will commence with thetransducer 18 in substantially a state that might exist during trackfollowing. Computer simulations have shown that such a final state ofthe transducer 18 can be achieved by using approximate values of theactual velocity of the transducer 18 as the accepted velocity at thebeginning of each time interval of each deadbeat interval.

In accordance with the above remarks, a second method of determining theaccepted velocity of the transducer 18, useful when the parametersavailable to the servo microprocessor 28 include the current through theactuator coil 38, is to adjust the average velocity of the transducer 18determined for the previous time interval for the acceleration of thetransducer 18 during that time interval. Thus, if the locations of thetransducer 18 at the beginning of a time interval Q is denoted X(Q) andthe duration of each time interval of each deadbeat interval is denotedT, the accepted velocity can be determined in accordance with theexpression

    Vacc(Q)=(1/T) X(Q)-X(Q-1)!+(1/2) Km I(Q) T                 (6)

where the first term is the average velocity at the temporal center ofthe previous time interval and the second term in the increase invelocity from the center of such previous time interval to the beginningof the present time interval using the expression for the accelerationof the transducer 18 in relation to the actuator coil current expressedabove in equation (1).

In disc drives which have neither a velocity transducer 56 nor a meansof providing the actuator coil current to the servo microprocessor 28,the accepted velocity can be determined from a series of successivelocations of the transducer 18 in accordance with the relationship

    Vacc=(1/T)  (5/3) X(Q)-(5/2) X(Q-1)+X(Q-2)-(1/6) X(Q-3)!   (7)

where the numerical coefficients of the successive positions of thetransducer 18 have been selected to include the effects of both theacceleration of the transducer 18 and time derivative of theacceleration, the so-called "jerk", of the transducer 18 in thedetermination of the transducer 18 accepted velocity.

Once the accepted velocity of the transducer 18 has been determined atthe beginning of the time interval, a check is made, decision block 84,to determine whether the settle has been completed. In the case in whichthe deadbeat constants used in the practice of the present invention aredetermined from a second order deadbeat strategy, two deadbeat periodswould be used to settle the transducer 18 on the target track and aconvenient criterion that the settle has been completed is that thedeadbeat interval index N, initialized to zero at the start of the seekthat terminates in the settle method of the present invention, hasadvanced to 2 as shown in FIG. 5. (As will be discussed below, thedeadbeat interval index is incremented at the end of each deadbeatinterval.) In circumstances in which the deadbeat constants used in thepractice of the present invention are determined from a higher orderdeadbeat strategy, the order of such strategy would be used in decisionblock 84.

In the settle method of the present invention, a deadbeat value of thecontrol signal outputted to the power amplifier 36 is determined in thefirst time interval of each deadbeat interval using the control equationfor the ideal deadbeat settle strategy that served as the basis fordetermining the deadbeat constants utilized in carrying out the presentinventive method. These values are then corrected in each time intervalto determine control signals that are outputted to the power amplifierto achieve a settle of the transducer 18 that closely follows an idealdeadbeat settle to bring the transducer to a location substantially onthe target track with a velocity of substantially zero.

To provide for the determination of the deadbeat value, the index Qidentifying each-time interval of each deadbeat interval is checked,step 86, in each execution of the routine illustrated by the flow chartof FIG. 5 for which the deadbeat index is less than the order of thedeadbeat strategy utilized to determine the deadbeat constants. If, aswill be the case for the initial execution of the routine, the timeinterval index Q is zero, the deadbeat value to be utilized for theentire deadbeat interval is calculated, step 88. In the case in whichthe inventive method utilizes second order deadbeat constants, Kp(DB)and Kv(DB), as described above, the deadbeat control value is determinedin accordance with the relation

    U(N)=-Kp(DB) X-Kv(DB) Vacc                                 (8)

where Kp(DB) and Kv(DB) are the second order deadbeat constantsdetermined as described above, X is the location of the transducer 18 atthe beginning of the first time interval of the deadbeat interval andVacc is the accepted value of the transducer velocity at the beginningof the first time interval of the deadbeat interval determined asdescribed above.

In the embodiment of the inventive method illustrated by the flow chartof FIG. 5, the control signal correction is selected to be zero for thefirst time interval of each deadbeat interval, step 90, so that thecontrol signal outputted to the power amplifier 36 in the first timeinterval of each deadbeat interval, determined by adding the intervalcorrection to the deadbeat value, step 92, will be the deadbeat value ofthe control signal. Such signal is outputted to the power amplifier 36,step 94, and the time interval index Q is incremented, step 96, topermit appropriate corrections to be determined for succeeding timeintervals of the deadbeat interval.

Following the increment of the time interval index, the value of theindex is compared to the number Qmax of time intervals in each deadbeatinterval, step 98, and, if the value of the index is less than Qmax, theservo microprocessor 28 returns to the start of the flow chart for thedetermination and output of the correction signal to the power amplifier36 for the next time interval of the deadbeat interval. (In a disc drivehaving a servo system as described in the aforementioned U.S. Pat. No.5,262,907, such return will be effected by terminating the presentinterrupt in which the above noted steps are carried out; in disc drivesin which the steps are carried out during execution of the main programof the servo microprocessor 28, a wait step (not shown) can be includedin the programming to establish a fixed duration for the time intervalsof the deadbeat intervals.)

In each time interval of each deadbeat interval following the first suchtime interval, the time interval index Q will be a value other thanzero. In such time intervals, the deadbeat value calculated in the firsttime interval is corrected to compensate for disturbances that wouldcause the transducer 18 to depart from a practicable settle if an idealdeadbeat strategy were used. To this end, in each succeeding timeinterval of each deadbeat interval, a projected velocity at thebeginning of the time interval for ideal deadbeat settle is calculated,step 100, and used to determine the time interval correction which, incombination of totality of time interval corrections for the remainingtime interval of all deadbeat intervals, will result in settle of thetransducer 18 on the target track that closely approximates idealdeadbeat settle.

The projected velocity is determined on the basis of the characteristicsof ideal deadbeat settle illustrated in FIGS. 2 and 3. Moreparticularly, as shown in FIG. 2, ideal deadbeat settle contemplates aconstant acceleration of the transducer 18 during each time interval ofeach deadbeat interval and such acceleration is determined by thedeadbeat value of the control signal in accordance with equations (1)and (2) above. Thus, the projected velocity; that is, the velocity thetransducer 18 would attain under ideal deadbeat settle at the beginningof the Qth time interval of the Nth deadbeat interval would be given by

    Vproj(Q)=Km Ki U(N) QT.                                    (9)

The time interval correction term for the Qth time interval is thendetermined, step 102, in accordance with the relation

    U(Q)=Kv(DB)  Vproj(Q)-Vacc(Q)!.                            (10)

Thus, the correction is a term that compensates for differences betweenthe velocity the transducer 18 would have for ideal deadbeat settle andthe velocity the transducer is caused to have by the presence ofdisturbances. The time interval correction is then added to the deadbeatvalue of the correction signal calculated in the first time interval ofeach deadbeat interval, step 92, and outputted to the power amplifier36, step 94, as described above.

The correction of the control signal outputted to the power amplifier 36in each time interval and the effect of the corrections during thesettle of the transducer 18 has been illustrated in FIGS. 2 and 3 for arepresentative case in which the actuator 40 is subjected todisturbances which will tend to cause the transducer 18 to overshoot thetarget track at the end of a seek; that is, to attain velocities thatare higher than velocities that would be attained in ideal deadbeatsettle. More particularly, FIG. 2 compares accepted velocities,indicated by circles 104, 106, 108, 110, 112, 114, 116, 118, 120 and 122determined at the start of the time intervals of the deadbeat intervalsto the ideal deadbeat velocity curve and FIG. 3 illustrates thecorresponding interval corrections that would be added to the deadbeatcontrol value in each of the time intervals. Thus, for example, for theaccepted velocity indicated by the circle 108 determined at the start ofthe Q=3 time interval of the N=1 deadbeat interval, the time intervalcorrection determined in accordance with equation (10) above would be anegative value indicated at 124 in FIG. 3 that would give rise to alarger braking force on the actuator 40 than would be exerted for anideal deadbeat strategy to compensate for tendency of the disturbance tocause the transducer 18 to overshoot the target. As indicated in FIG. 2,the totality of the time interval corrections results in a trajectorythat terminates in a velocity of the transducer 18 at the end of thesettle that is substantially zero and a location, compared to the finallocation of zero for ideal deadbeat settle, that differs from thelocation of zero for ideal deadbeat settle only by the difference of theareas under the deadbeat velocity curve and the trajectory of thetransducer 18 indicated by the accepted velocity points. Thus, at theend of the final time interval of the final deadbeat interval, thetransducer 18 will be located substantially on the target track with avelocity of substantially zero.

Returning to FIG. 5, each time the time interval index Q reaches themaximum value Qmax, the deadbeat interval index N is incremented, step126, and the time interval index Q is reset to zero, step 128, so thatthe settle of the transducer 18 will be effected in N deadbeat intervalsand will then terminate at the decision block 84. Following thetermination of the settle, the servo microprocessor 28 exits to aconventional track following mode of operation.

FIG. 6 illustrates a modification of the invention which is useful indisc drives in which the time constant for the power amplifier is largein comparison with the duration of the time intervals of the deadbeatintervals. In such case, the actuator coil current can differsignificantly from the value determined by equation (2) for several timeintervals at the beginning of each deadbeat interval to result in afinal state that differs appreciably for the ideal deadbeat final state.FIG. 6 illustrates the manner in which the effect of a large poweramplifier time constant can be compensated.

Most of the steps of the embodiment of the invention illustrated in FIG.6 are the same as the steps of the embodiment illustrated in FIG. 5 sothat only the differences need be noted for purposes of the presentdisclosure. Accordingly, steps that are identical to steps carried outin the embodiment illustrated in FIG. 5 have been given the samenumerical designations used in FIG. 5 and will not be further discussed.

In the embodiment illustrated in FIG. 6, it is contemplated that thedisc drive will include a capability for providing the actuator coilcurrent to the servo microprocessor 28 and such current is inputted,step 130, following input of the transducer position error at step 80.The actuator coil current then provides a basis for determining a secondtime interval correction that is added to the time interval correctionsshown in FIG. 5, such time interval corrections for time intervals otherthan the first time interval of each deadbeat interval constituting afirst time interval correction determined as described above for FIG. 5.Thus, the embodiment shown in FIG. 6 contemplates the determination of afirst time interval correction ΔU1 in the step 102 and a second timeinterval correction in a succeeding step 132 in time intervals otherthan the first time interval of each deadbeat interval. Similarly, sincea long time constant for the power amplifier will have the greatesteffect on transducer settle in the first time interval of each deadbeatinterval, a correction is determined for each of the first timeintervals of the deadbeat intervals as well. It will be useful toinitially consider the correction made in the first time interval ofeach deadbeat interval.

As will be appreciated by those of skill in the art, the effect of along time constant for the power amplifier 36 will be to delay changesin the current that is passed through the actuator coil 38 when a changein the control signal outputted by the servo microprocessor 28 occurs.While such a delay will have a negligible effect on the settle of thetransducer 18 for small changes in the control signal from one timeinterval of a deadbeat interval to another time interval of the samedeadbeat interval, the relatively large change in the control signalthat can be expected at the beginning of each deadbeat interval will notbe reflected in the acceleration of the transducer 18 for a time thatdepends on the amplifier time constant. More particularly, a change inthe current arising from a step change in the control signal willgenerally be an exponential function of time.

To correct for this exponential change, it is recognized that the valueof the current inputted to the servo microprocessor at the beginning ofthe first time interval of each deadbeat interval will be the value ofthe current that was determined by the control signal for the last timeinterval of the previous deadbeat interval or the last time intervalduring velocity control of the transducer 18. Such current willcorrespond to a nominal control signal value via the relationshipexpressed in equation (2). Such nominal value is calculated in the firsttime interval of each deadbeat interval, step 134, and utilized in thedetermination of a time interval correction in all time intervals of thedeadbeat interval. More particularly, the nominal value of the controlsignal is subtracted from the deadbeat value of the control signaldetermined in the first interval of each deadbeat interval to obtain achange in control signal that corresponds to the difference betweenideal deadbeat currents for two successive deadbeat intervals or betweenthe last velocity control current and the ideal deadbeat current for thefirst deadbeat interval. Accordingly, the compensation can be effectedby multiplying the difference between the deadbeat value for thecorrection signal and the nominal control signal value by a discretetime interval dependent function that corresponds to exponential decayof the actuator coil current from one value to another; that is, adiscrete representation of the function

    K(t)-K(0)exp (-t/Tc),                                      (11)

where Tc is the power amplifier time constant, and selecting K(0) to bea weighting value that will cause the area under the curve expressed byequation (11) from t=0 to t=T, the duration of a time interval of adeadbeat interval, to be K(0) T. Thus, in the first time interval ofeach deadbeat interval, a time interval correction

    ΔU=K(0)  U(N)-Unom!                                  (12)

is generated, step 136, and added to the deadbeat value of the controlsignal at step 92 for output to the power amplifier 36 during the firsttime interval of the deadbeat interval.

In succeeding time intervals of each deadbeat interval, the form ofequation (11) is utilized to determine the second time intervalcorrection that has been referred to above. Specifically, since the timeintervals of the deadbeat intervals are of the same duration T, thesecond time interval corrections calculated at step 132 can be found byevaluating the exponential function of equation (11) at successivemultiples of the time interval T. Thus, the second time intervalcorrections for time intervals of the deadbeat intervals are generallygiven by

    ΔU.sub.2 =K(Q)  U(N)-Unom!,                          (13)

    where

    K(Q)=K(0) exp (-Q T/Tc).                                   (14)

These time interval corrections are added, step 138, to the first timeinterval corrections calculated at step 102 to obtain time intervalcorrections which, in combination with each other and with the deadbeatvalues of the control signal, will cause the transducer 18 to settle tothe target track in a manner that closely approximates ideal deadbeatsettle.

It will be clear that the present invention is well adapted to carry outthe objects and attain the ends and advantages mentioned as well asthose inherent therein. While embodiments have been described forpurposes of this disclosure, numerous changes may be made which willreadily suggest themselves to those skilled in the art and which areencompassed in the spirit of the invention disclosed and as defined inthe appended claims.

What is claimed is:
 1. In a disc drive of the type having a rotatabledisc and a controllably positionable actuator adjacent the disc, thedisc including a surface upon which a plurality Of tracks are defined,the actuator having a transducer for reading data from and writing datato the tracks, the disc drive further having a power amplifier fordriving the actuator to accelerate the transducer radially across thedisc surface in response to a control signal and a servo system,including a servo microprocessor, for generating the control signal inrelation to measured parameters indicative of at least the location ofthe transducer with respect to a target track, a method for settling thetransducer onto the target track during which the transducer iscontrollably decelerated from a non-zero initial velocity at a selecteddistance from the target track to a substantially zero terminal velocityover the target track, the method comprising the steps of:inputting saidparameters to the servo microprocessor in each of a plurality of timeintervals, wherein said time intervals are grouped into a plurality ofdeadbeat intervals; determining a deadbeat value of said control signalfrom said parameters in the first time interval of each deadbeatinterval; determining an interval correction to said deadbeat value foreach time interval of each deadbeat interval; generating the controlsignal in relation to the sum of the deadbeat value and the intervalcorrection; and outputting the control signal from the servomicroprocessor to the power amplifier.
 2. A method for settling atransducer on a target track on the surface of a rotating disc of a harddisc drive of the type having an actuator whereon the transducer ismounted, a power amplifier for driving the actuator to accelerate thetransducer radially across the disc surface in response to a controlsignal and a servo system, including a servo microprocessor, forgenerating the control signal in relation to measured parametersindicative of at least the location of the transducer with respect tothe target track, comprising the steps of:inputting said parameters tothe servo microprocessor in each of a plurality of time intervals,wherein said time intervals are grouped into a plurality of deadbeatintervals; determining a deadbeat value of said control signal from saidparameters in the first time interval of each deadbeat interval;determining an interval correction to said deadbeat value for each timeinterval of each deadbeat interval, comprising the steps of:selecting atime interval correction of zero for the first time interval of eachdeadbeat interval; and for each remaining time interval of the deadbeatinterval, the steps of:determining an accepted value of the transducervelocity at the beginning of the time interval from said parameters;determining a projected value of the transducer velocity at thebeginning of the time interval from the deadbeat value of the controlsignal; and determining the time interval correction in proportion tothe difference between the projected value of the transducer velocityand the accepted value of the transducer velocity; generating thecontrol signal in relation to the sum of the deadbeat value and theinterval correction; and outputting the control signal from the servomicroprocessor to the power amplifier.
 3. A method for settling atransducer on a target track on the surface of a rotating disc of a harddisc drive of the type having an actuator whereon the transducer ismounted, a power amplifier for driving the actuator to accelerate thetransducer radially across the disc surface in response to a controlsignal and a servo system, including a servo microprocessor, forgenerating the control signal in relation to measured parametersindicative of at least the location of the transducer with respect tothe target track and the output of the power amplifier, comprising thesteps of:inputting said parameters to the servo microprocessor in eachof a plurality of time intervals, wherein said time intervals aregrouped into a plurality of deadbeat intervals; determining a deadbeatvalue of said control signal from said parameters in the first timeinterval of each deadbeat interval; determining an interval correctionto said deadbeat value for each time interval of each deadbeat interval,comprising the steps of:in the first time interval of each deadbeatinterval the steps of:determining a nominal control signal valuecorresponding to the power amplifier output inputted to themicroprocessor in said first time interval of the deadbeat interval; anddetermining the interval correction in relation to the differencebetween said deadbeat value of the control signal and said nominalcontrol signal value; and in each remaining time interval of thedeadbeat interval, the steps of:determining an accepted value of thetransducer velocity at the beginning of the time interval from saidparameters; determining a projected value of the transducer velocity atthe beginning of the time interval from the deadbeat value of thecontrol signal; determining a first time interval correction inproportion to the difference between the projected value of thetransducer velocity and the accepted value of the transducer velocity;determining a second time interval correction in relation to thedifference between said deadbeat value of the control signal and saidnominal control signal value; and adding the first and second timeinterval corrections to obtain the interval correction for the timeinterval; generating the control signal in relation to the sum of thedeadbeat value and the interval correction; and outputting the controlsignal from the servo microprocessor to the power amplifier.